After a materialized view is created for an SQL statement, the SQL statement is rewritten to be queried through the materialized view when the SQL statement is executed. If the rewrite cache function is enabled for materialized views, the rewritten SQL statements will be saved to the cache (a maximum of 10,000 records can be saved by default) after the SQL statement is executed for multiple times. When the SQL statement is executed within the cache validity period (24 hours by default), the system obtains the rewritten SQL statement from the cache instead of rewriting the SQL statement.

You can add user-defined parameters rewrite.cache.timeout and rewrite.cache.limit to a compute instance to set the cache validity period and the maximum number of rewritten SQL statements that can be saved.

When a new materialized view is created or an existing materialized view is deleted, the cache becomes invalid.
If the materialized view associated with a rewritten SQL query in the cache becomes invalid or is in the Refreshing status, the rewritten SQL query will not be used.
When the cache is used, the executed SQL query cannot be changed. Otherwise, it will be treated as a new SQL query.
A maximum of 500 materialized views can be rewritten for SQL queries. That is, if the materialized views used during SQL rewriting are included in the 500 materialized views, the views will be rewritten. Otherwise, the views will be executed as common SQL statements. You can refer to System level to add user-defined parameter hetu.select.top.materialized.view to compute instances to change the number of materialized views that can be used.

Enabling Rewrite Cache for Materialized Views

Session level:
Run the set session rewrite_cache_enabled=true command on the HetuEngine client by referring to Using the HetuEngine Client.

Enabling the materialized view rewriting capability at the system level:

Log in to FusionInsight Manager as a user who can access the HetuEngine web UI.
Choose Cluster > Services > HetuEngine to go its service page.
In the Basic Information area on the Dashboard page, click the link next to HSConsole WebUI. The HSConsole page is displayed.
Check whether the status of the instance to be operated is STOPPED. If not, change the status to STOPPED.

Locate the row that contains the target instance, click Configure in the Operation column, and add the following customized parameters:

Parameter	Value	Parameter File	Description
rewrite.cache.enabled	true	coordinator.config.properties	Enable the rewrite cache function for materialized views.
rewrite.cache.timeout	86400000	coordinator.config.properties	Change the validity period of the rewrite cache. If this parameter is left blank, 86400000 is used by default. The unit is ms.
rewrite.cache.limit	10000	coordinator.config.properties	Modify the upper limit of the rewrite cache. If this parameter is left blank, 10000 is used by default.

After the parameters are added, select Start Now and click OK.

Configuring Caching of Materialized Views

Enabling Rewrite Cache for Materialized Views